Green’s Dictionary of Slang

showy, given to excess, esp. in dress.

a.1790		C. Dibdin ‘The Dustman’ in Collection of Songs II 77: I’m dashing Dick the dustman.
1821		‘A Real Paddy’ Real Life in Ireland 117: What dashing belle is that making love to a young officer?
1824		Northampton Mercury 24 Jan. 4/1: At the sign of — in Long Acre [celebrated] not merely for the prime liquors it contained, but for [...] the dashing Miss — , [...] the barmaid.
1830		‘Chapter of Donkies’ in White Friars Collection 7: I’m call’d dashing Dick, the Tothill-fields lad.
1841		‘Ax My Eye’ Dublin Comic Songster 101: I sports a hellish swellish / Coat [...] and a slashing dashing, / Stunning pair o’ pickling tubs.
1856		Miss Edgeworth Almeria 292: She was astonished to find in high life a degree of vulgarity of which her country companions would have been ashamed: but all such things in high life go under the general term dashing. These young ladies were dashers [F&H].
1861		(con. 1840s–50s) H. Mayhew London Labour and London Poor III 211/2: There was a good many dashing young shop chaps came and had their portraits taken.
1873		Sl. Dict.